Huge Satellite Poses 150-Year Threat of Space Debris

BREMEN, Germany - In
three years, the European Space Agency will become the owner of what is
possibly the most dangerous piece of space debris circling the Earth for the
next 150 years: the 17,636-pound Envisat Earth
observation satellite.

The space agency will
take control of the Envisat satellite, which has been
extended to 2013 and appears to set records wherever it goes.

Launched in 2002, Envisat was the biggest non-military Earth observation
satellite
ever built. At $2.9 billion in today's dollars, it is one of the most
expensive. Its mission is viewed as a success by its users, all the more so
insofar as the original five-year mission has been stretched to 11 years.

And now, once in
retirement and in a near-polar orbit at 486 miles (782.4 km) in altitude, Envisat will become what space debris experts say is a huge
problem that will not go away for about 150 years. That is how long it will
take for Envisat, given its orbit and its
area-to-mass ratio, to be gradually pulled into the Earth's atmosphere. [Most Memorable Space
Debris Events]

Big satellite, big
threat

Space debris experts
attending the 38th Congress of the Committee on Space Research (Cospar) here July 18-25 said an event last January brought
home just how much of a threat to the low-Earth orbit environment Envisat will be. That was when the U.S. Space Surveillance
Network warned ESA that a 3,306-pound (1,500-kg) upper stage from a Chinese
rocket was bearing down on Envisat and that the
"conjunction assessment" pointed to a likely impact.

HeinerKlinkrad,
head of ESA's space
debris office at ESOC, said here July 21 that a post-event analysis showed
that the Chinese stage probably would have collided with Envisat
if the avoidance maneuver had not been done. Such maneuvers will not be
possible once Envisat is retired.

ESA officials, more
accustomed to speaking of Chinese, Russian or American debris issues, are
uncomfortable discussing the danger that Envisat
represents, especially since the agency has showcased the satellite as a major
success.

Klinkrad is no exception. But
he did say that if the collision with the spent Chinese upper stage had
occurred, it likely would have polluted a highly used portion of low Earth
orbit with 10 times as much junk as what was caused by the 2009 collision of an
operational Iridium communications satellite with a retired Russian Cosmos
spacecraft. That event occurred at about the same altitude where Envisat flies.

Huge satellite debris
risks

Envisat's 17,636-pound (8,000-kg)
mass alone would be enough to put it onto the top tier of space debris threats, even though there
are nearly a dozen spent Russian rocket upper stages that weigh as much as or
more than Envisat.

But Envisat's configuration in orbit makes it a unique concern,
even beyond its weight. The satellite's in-orbit size is 26 meters by 10 meters
by five meters. Its suite of observing instruments uses a small farm of
antennas that likely have become more fragile after a decade in orbit. That
means that even a small piece of debris ? pieces too small to be cataloged by
the Space Surveillance Network ? could cause what debris specialists refer to
as a "fragmentation event" that would produce its own population of
space garbage.

An analysis of the
space debris environment at Envisat's orbit suggests
that there is a 15 percent to 30 percent chance of the satellite colliding with
another piece of junk during the 150 years it remains in orbit. But that
likelihood is based on the current population of space debris in low Earth
orbit remaining constant during the period ? a scenario no one believes is
remotely possible.

NASA's assessment is
that even if the world's space-faring nations stopped all launches today, the
debris population in low Earth orbit would continue to increase for half a
century as collisions increase the population faster than the natural cleansing
effect of atmospheric drag.

Envisat program managers say
that for many reasons, including industrial policy and overall program costs, Envisat was built with a fuel tank used by the French Spot
4 optical Earth observation satellite, whose launch weight was less than half
of Envisat's.

With such a small fuel tank, any attempt to
bring the satellite's orbit down to where it could re-enter the atmosphere
would have meant retiring Envisat just a few months
after its launch.

ESA officials insist
that the international guidelines on disposal of debris were not in force when Envisat was designed.

Too
big to fall?

Further complicating Envisat's post-retirement status is that the satellite is
too big to be allowed to drift into the Earth's atmosphere. Emerging European
space safety regulations require that space hardware big enough that large
chunks will survive atmospheric re-entry must be guided so that they enter the
atmosphere over uninhabited areas, such as the South Pacific.

Jurgen Starke of Astrium Space Transportation, whose company is under ESA
contract to study debris removal technologies, said current French law ? France
has jurisdiction over Europe's Guiana Space Center spaceport ? requires that
the likelihood of re-entering the atmosphere over a populated area not exceed
1-in-10,000.

Starke said here July
21 that Astrium is still uncertain whether, in Envisat's case, it would be better to raise the satellite
to an orbit that currently is unused, or to guide it into the Earth's atmosphere
over the Pacific Ocean. In either case, a mission would cost several hundred
million dollars including launch, he said. For the moment, neither scenario is
likely to be approved by ESA governments.